1. Field of the Invention
The present invention relates generally to semi-conductor processing and optical coatings. Particularly, the present invention relates to physical vapor deposition onto substrates.
2. Description of the Prior Art
Electron beam evaporation is commonly used to coat wafers with a thin metallic layer in a process known as metallization. Generally, in typical silicon wafer fabrication, the metallic layer deposited is then etched to form circuit traces of an integrated circuit. For high frequency integrated circuits, gallium arsenide (GaAs), indium phosphide (InP) and numerous alloys between the two and similar electro-optical materials are now typically utilized as a substrate. Some metals, however, form a superficial oxide on their surface that is known in the art as the “skin effect.” In circuits where high frequency power is used, this is a problem. This is critical especially for IC chips used in cellular devices since the skin effect increases power consumption.
Gold is often used as the conductor of integrated circuits because, as a passive metal, gold will not form a superficial oxide. Unfortunately, the use of gold presents other problems. The problems arise when the materials used as a substrate are ones other than silicon such as the gallium arsenide previously mentioned. There are two problems with depositing a gold layer directly upon a GaAs substrate. First, the gold will leach into the substrate. Second, the gold will not adequately adhere directly to the substrate. Therefore, in order to prevent the gold from leaching into the substrate, a diffusion barrier of palladium or platinum separates the gold from the GaAs. Additionally, an adhesion layer of titanium or chromium is deposited upon the GaAs substrate between the substrate and the diffusion barrier in order to make the gold, and the diffusion barrier adhere to the substrate. These barrier and adhesion layers must typically be very thin yet very uniform.
Unlike gold circuit traces on a silicon substrate, the gold circuit traces cannot be etched away from the GaAs substrate in a typical etching process because the etchant would remove the adhesion layer and diffusion barrier thus freeing the circuit trace from the substrate. This is a clearly undesirable consequence. Therefore, the gold circuit traces are typically made according to a “lift-off” process, all as is well known in the art. To use this process, a source of metal to be deposited must achieve a trajectory as close to 90 degrees with respect to the substrate surface as possible. This is referred to as orthogonal deposition and the optimal resultant coating is referred to as a “lift-off” coating or as zero step coverage. A commonly used method of physical vapor deposition in lift-off processes is electron beam evaporation. In practical applications where multiple wafers must be precisely coated by a single source, this requires complex machinery with specific setups for specific power levels and materials. Examples of these complex systems are disclosed.
U.S. Patent Application Publication No. 2003/0180462 (2003, Chang et al.) discloses a planetary lift-off vapor deposition system. The system incorporates multiple domes mounted in a square or rectangular vacuum chamber with a planar top wall and a planar bottom wall. The multiple domes rotate about the source centerline axis and about another second axis of rotation in order to assure an even coating on the target wafers and to utilize a larger percentage of material evaporated from the source compared to single dome systems. The system is configured to produce orthogonal lift-off coatings without the use of a uniformity mask.
The advantage of the Chang et al. system is the use of multiple domes in a planetary configuration to achieve a process that is less sensitive to process variations such as evaporant material, power level, beam position, and the like without the use of a uniformity mask. Although the Chang device provides for some improvement in collection efficiency over a single-dome system, the disadvantage of the Chang et al. system is there is still waste of evaporant material on surfaces other than the target wafers.
U.S. Pat. No. 3,858,547 (1975, Bergfelt) discloses a coating machine having an adjustable rotation system. The coating machine has a cylindrical-shaped vacuum chamber with a planar top wall and a planar bottom wall. At least one coating source is included therein. The coating source is located in a resistance heated boat arranged in a 12-inch circle. A plurality of spindle assemblies is mounted in the chamber with each of the spindle assemblies having a rotatable spindle. A substrate holder is carried by each spindle and is adapted to carry a substrate in such a manner that it is adapted to receive coating material from the coating source. Means is provided for rotating the spindle assemblies about the source and for rotating the spindles about their own axes at the same time they are being rotated about the source. Additionally, means is provided to permit adjustment of the spacing of the spindle assemblies from the center of rotation about the source. In addition, means is provided to permit adjustment of the angle of the spindle with respect to the coating source to thereby adjust the angle of incidence of the vapor coating stream with respect to the substrates carried by the spindles.
The disadvantage of the Bergfelt device is that the spindle assemblies must be adjusted depending on where the coating source boat is located in the 12-inch circle. This reduces throughput of the number of wafers because of the additional set up time required. Although the Bergfelt device offers an improved efficiency over the single-dome system, it still suffers from a waste of evaporant material on surfaces other than the target wafers on the spindle assemblies.
U.S. Pat. No. 3,643,625 (1972, Mahl) discloses a thin-film deposition apparatus having a holder and a plurality of racks. Each of the racks has a surface that has the conformation of a portion of the surface of a sphere. A structure is provided for rotatably mounting the racks on the holder in such a manner that the surfaces of the racks lie generally on the surface of a common sphere. A drive mechanism is provided for rotating the holder upon an axis centrally disposed with respect to the racks and for rotating the racks about their own axes of rotation with respect to the holder. A source of the material appears to be positioned on approximately the surface of the sphere or further from the racks.
A disadvantage of the Mahl device is that locating the source of material on or further than the surface of the sphere requires a much larger size of the vacuum chamber. By using a larger vacuum deposition chamber, a larger amount of exposed surface area is available for receiving the evaporated source material that is not deposited on the surfaces intended to receive the film coating. Furthermore, the wafers are not orthogonal to the evaporant source, thus making it unacceptable for use with a “lift-off” process.
Therefore, what is needed is an electron beam coater capable of the orthogonal deposition necessary for lift-off applications that is less sensitive to process variations such as evaporant material, power level, beam position, and the like. What is also needed is an electron beam coater that is more efficient.